1. Field of the Invention
This invention relates to a method and structure for fabrication of infrared focal plane arrays (IRFPAs) with high quality photodiodes.
2. Brief Description of the Prior Art
Prior art techniques for fabrication of infrared focal plane arrays have utilized planar processing (VIP, vertical diodes) and bump bonded techniques (BLIMP, DLHJ). The planar processing is more appropriate for mass manufacturing techniques, however temperature restrictions after hybridization prevent use of preferable passivation methods. Bump bond techniques require difficult aligned hybridization with extremely tight mechanical tolerances. The bump bond techniques generally do not permit thinning and are not stable to temperature cycling, especially with large area formats. Both planar processing and bump bonded techniques display long process flows. It follows that a simpler process flow that can provide the same or similar results will provide an economic advantage.
In accordance with the present invention, there is provided a very rapid, simple and highly manufacturable fabrication process for IRFPAs with high quality photodiodes.
Briefly, in accordance with the present invention, advantage is taken of the property whereby vacancy doped or mobile extrinsic dopants in mercury cadmium telluride, preferably gold doped p-type mercury cadmium telluride (HgCdTe), when damaged in the lattice, forms an n-type region in the vicinity of the damage, thereby forming a diode with the adjacent p-type region. Accordingly, in accordance with the present invention, photodiodes are formed in the HgCdTe by pressing a surface of the HgCdTe against a readout integrated circuit with a damage creating feature on the surface of the integrated circuit, such as an electrically conductive bump that preferably also forms an ohmic contact with the HgCdTe. The bumps, which are preferably of tungsten or tin coated tungsten or tungsten coated with a mercury amalgam that will give up the mercury to the surrounding HgCdTe under conditions of elevated temperature or the like, are disposed on the surface of the readout integrated circuit and contact the surface of HgCdTe under pressure and preferably elevated temperature, in the range of 100xc2x0 to 140xc2x0 C., preferably about 110xc2x0 C., to damage the surface of the HgCdTe in the immediate vicinity of the contact. The damaged area of the HgCdTe is thereby converted from p-type to n-type to provide a diode. Thereby, ohmic contact as well as conductivity type conversion and diode formation is provided at the region of the damage on the HgCdTe surface in a single step. By arranging damage creating or inducing elements, such as electrically conductive bump contacts, preferably having a sharp edge which first contacts the HgCdTe surface, on the surface of the integrated circuit in an appropriate pattern, a pattern of diodes is formed in the HgCdTe with a conductor therefrom directly to the readout integrated circuit. Bumps can be formed, for example, in accordance with the procedures as set forth in xe2x80x9cPhysical properties of thin-film field emission cathodes with molybdenum conesxe2x80x9d, Journal of Applied Physics, Vol. 47, No. 12, December 1976, pp 5248-51 and the references cited therein. The created bondline also allows the HgCdTe to be thinned to allow temperature cycling to 77xc2x0 K without damage with retention of most of the advantages of planar processing. Furthermore, the architecture of the present invention is amenable to both staring and scanning IRFPAS.